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Community and Q&A

Modified flash-and-batt

Dave Frank | Posted in Energy Efficiency and Durability on

Is it reasonable to do a combination of closed-cell spray foam in the unfinished attic rafters and batts on the ceiling flats?

Reasoning for this is to (a) get HVAC in the attic within the envelope where rafters can’t be properly vented, and (b) still have insulation on top of the flat ceiling drywall because there is some ceiling-radiant heat. And there’s the added benefit of getting some of the R value from a cheaper product.

Does the R38 code requirement allow the insulation to be in two different places as long as it totals the appropriate R-value?

Also, should the minimum depth of spray foam be 3 inches to control condensation against the roof sheathing? (Am in climate zone 5)


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  1. GBA Editor
    Martin Holladay | | #1

    You can't do it, because the International Residential Code (IRC) specifically requires that (if you take the flash-and-batt approach) "the air-permeable insulation shall be installed directly under the air-impermeable insulation."

    The code provision can be found here: R806.5 Unvented attic and unvented enclosed rafter assemblies.

    This section of the 2012 IRC (section R806.5.3) reads, "5.3. Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation."

    So, if you want to meet the code requirement for R-38 insulation with a combination of closed-cell spray foam and fiberglass batts, then the two types of insulation must be in contact with each other.

    Once you have met the code provision for R-38 insulation between your rafters, however, you are still free to install fiberglass batts on your attic floor if you want.

  2. Aj Builder, Upstate NY Zone 6a | | #2

    Martin, read your post. Direct contact then directly under, not direct contact. contact is only mentioned once in what you quoted then you go on and assume contact twice.

    If you stand directly under me and I drop my hammer, it will hit you.

    So the language is open to interpretation IMO. Not saying whether the idea is good... Not against someone seeing if the idea works or not.

  3. Expert Member
    Dana Dorsett | | #3

    AJ: Seriously?

    "The air-permeable insulation shall be installed directly under the air-impermeable insulation. "

    Even a first-year weasel lawyer would have a tough time concocting an interpretation that the air-permeable insulation didn't need to be in VERY close proximity, even though code does not demand a compression-fit contact)with the air-impermeable foam. :-)

    Code is one thing, the real building science is another. Whether it creates a problem depends a lot on how the intervening space is used (if at all), and the ratio of foam-R/fiber-R at the attic floor. Air tightness at the ceiling plane below the floor becomes hyper-critical if the R-ratio isn't super robust- you can't go with the IRC minimums for the air-impermeable layer and get away with it. Foam installers are fond of removing all floor insulation when foaming the roof deck (presumably to meet code), but that isn't truly necessary in many (or even most) cases in climate zones 5 or lower.(Good luck arguing that with the code inspectors though.)

  4. Atticus LeBlanc | | #4

    Can anyone speak to the science behind this reasoning? It wasn't that long ago that codes required vented attics after all. I can't figure out why the distance between R12 closed cell foam on the attic deck/rafters and sidewalls and fluffy R-38 on the ceiling would have an impact on condensation if any; and the insulation to the heated cavity would still be more than required... at least for me in Zone 3.

  5. Atticus LeBlanc | | #5

    Also, could you not simply use baffles and eave vents above the closed cell to meet code and give you a vented attic?

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Could you not simply use baffles and eave vents above the closed cell to meet code and give you a vented attic?"

    A. If you insulate between your rafters with spray foam, and you provide a ventilation gap between the top of the insulation and the underside of the roof sheathing, you haven't created a vented attic. The attic is sealed and unvented. All you have done is created a ventilation channel above your cathedral-ceiling insulation.

    Q. "Can anyone speak to the science behind this reasoning [the reasoning for a code provision that requires that the two different types of insulation used for a flash-and-batt job in a roof assembly above an attic to be touching rather than separated by several feet]?"

    A. If the attic were perfectly sealed -- few attics are -- it's probable that the proposed (illegal) assembly would work well. But if the "sealed" attic had air leaks -- infiltration and exfiltration -- you wouldn't get the R-value you expect, and you would have opportunities for condensation or moisture issues.

  7. Aj Builder, Upstate NY Zone 6a | | #7

    To the jury I quote said language and let you use your wise minds in determining the meaning of "directly under"... I rest my case.

    The writers of this regulation can easily fix the language by adding directly under and "in contact with" , contact is not mentioned. Contact is not mandatory.

    Codes aren't perfect... as some above have stated... in all actuality there are many assemblies that work that one person will say not to do and another one will do and works just dandy. And then there are assemblies that are pushed here such as putting rigid foam on OSB properly yet I have taken apart some rotted messes built as such.

    This site is not for rigid foam on the interior yet it works great on the interior. And harmlessly as to rot. (Inside use when brought up is brushed aside and outside use is advocated. Well hold your horses folks... postings and blogs of late mention that warm polyiso is good and cold batts is good... does that mean ten years of advocating the exterior foam use needs to be nixed? Time to eat crow? Time to ADMIT all is not well in the green advocacy world?

    This site poo poos Bruce Brownell because he talks in terms of "equivalent R values" yet there so many times here R value is mentioned of a wall where the R value is actually way lower due to the fact that it is discontinuous and has a multitude of other problems... So R value folks is VERY MUCH a slippery slippery number to point of losing real value if not fully explained EVERY time it is used and misused not just when Bruce adds the word equivalent. I understand Bruce far more than the way R is tossed around in others contexts.

    This site talks up rigid insulution... and as time goes by... you fail to keep mentioning that a huge problem with rigid foam is air leaks past it. Rigid insulation is easily installed poorly much easier to install poorly and do harm than fiberglass batts that are poo pooed here.

    Construction is not a cake walk. It can be. But so much of what is talked about is like doing a bit of heart surgery after reading a heart surgery blog.

    Last point, Anybody wanting to start to insulate with rigid foam would do well to learn how to do it right and in a way that will not lead to problems (Bruce Brownell and his builders could show you.) And that goes for spray foam and combinations of insulations. Standard batt insulation though thought the least of is about the easiest for a DIYer.

    Don't complain about a Brownell home until after you have been in a few. Then write a blog.

  8. Expert Member
    Dana Dorsett | | #8

    The only jury you have to convince is your code inspector when parsing the letter of the code.

    I don't know of anybody who discounts Bruce Brownell, but his "equivalent-R" terminology presumes a cavity-fill-R framing lumber point of reference, and takes some interpretation. It's less ambiguous to use "whole-wall R", or "average U-factor", which are agnostic of construction type, no interpretation needed. A 2x4 R13 wall with OSB sheathing, wood siding and half-inch gypsum runs about R10 whole-whole wall, as does an 8" CMU wall with 2" of EPS and fiber cement on one side, half-inch gypsum on the other. But calling the CMU wall with the R8 continuous foam "R13 equivalent" isn't very enlightening without spelling out the framed-wall paradigm it refers to.

    And that's even before you get into the dynamic thermal mass effects of the CMU wall. :-)

  9. Aj Builder, Upstate NY Zone 6a | | #9

    Dana, you are the exception. keep up your very detailed postings and clarifications. This site jumped an order of magnitude to the positive with your presence. And as to being "green" it jumped the opposite direction with the absence of natural green "no corporations) green postings and blogs.

    GBA.... you need a natural green tab and editor. Martin I know you live your whole life closer to this ideal than Taunton... push for some natural green content on a regular basis.

    $8000 systems to ventilate a small home in the year 2014.... not so green if you ask me unless counting the green backs spent. Great home and system for the upper upper class... not gonna be installed in any paid by the hour folks home which is what, most of Merica land of the well off fat and happy on corporate fast food... thanks... can I get Zehnder with my order of fries please?

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